The purpose of this project is to determine the role played by tropomyosin in the organization of the actin cytoskeleton in normal and transformed rat kidney cells in culture. When the project was begun, it was known that tropomyosin associated periodically with bundles of actin filaments, termed "stress fibers," in normal cells. The initial purpose was to follow up on observations that tropomyosin was greatly reduced or absent in transformed cells lacking stress fibers. In order to test the hypothesis that tropomyosin is essential for the organization of stress fibers, tropomyosin purified from chicken gizzard or from normal cells was injected into transformed cells. Thus far, the injection of tropomyosin alone has not stimulated transformed cells to organize stress fibers. It has been demonstrated, however, that the injection of affinity-purified, antitropomyosin antibodies into normal cells can disrupt stress fibers and cell shape. A careful analysis of the tropomyosin content of normal and transformed cells has been carried out, and it is now clear that tropomyosin is present as five subunits in normal cells with a pair of subunits of apparent molecular weight of 30 kilodaltons being much more abundant than the others of higher weight. The latter are reduced or missing in transformed cells. One of the 30 kilodalton subunits is phosphorylated in cells grown with 32P in the medium. In the course of this work, a new observation has been made that tropomyosin can be localized in ruffling lamellae at the edges of both normal and transformed cells. Electron microscopic observations indicate that these lamellae also contain bundles of well-defined F-actin filaments. (L)